Proposed resolution:

This wording is relative to N4700.

1. Change in [default.allocator] as indicated:

   namespace std {
     template <class T> class allocator {
     public:
       using value_type = T;
       using propagate_on_container_move_assignment = true_type;
       using is_always_equal = true_type;
       constexpr allocator() noexcept;
       constexpr allocator(const allocator&) noexcept;
       constexpr template <class U> allocator(const allocator<U>&) noexcept;
       ~allocator();
       T* allocate(size_t n);
       void deallocate(T* p, size_t n);
     };
   }
   

[ 2018-3-17 Adopted in Jacksonville ]

[ 2017-11-25 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]

std::allocator's constructors should be constexpr. It's expected to be an empty class as far as I know, so this should impose no implementation burden, and it would be useful to permit guaranteed static initialization of objects that need to hold a std::allocator, but don't have to actually use it until after construction.